Since the formation of the U.S. Navy by the Continental Congress on October 13, 1775, soldiers and personnel have been recruited into its ranks to defend from enemies both foreign and domestic. This recruitment has now been extended to the microbial population. The manipulation and duplication of bacterial metabolism and function is nothing new: It has formed some of the most prolific advances in medicine and chemistry over the past two centuries.

The biogenic production of gases such as methane, carbon monoxide, CO2 and hydrogen has shaped the present form of our ecosystem, starting from the earliest stages of life on Earth. Biogenic gases are viable replacements for fossil fuels in both heat and power applications. Hydrogen can be produced from carbohydrates by several strains of bacteria; of particular interest are members of the Clostridia genera, which can generate hydrogen from soil and wastewater samples.

An illustration showing how bacteria can be utilized for the complete control of an autonomous sensor.

These advances have led to energy-harvesting microbial fuel cells, which take nutrients from the environment and turn them directly into electricity. Biogenic gases integrated with energy-harvesting microbial fuel cell technology will allow water column sensors and communication relays to have longer deployments, an essential quality when addressing security issues involving coastlines, seaports and choke points.

Beyond creating energy, one use of bacteria is to make existing systems use less power. A potential use could be on distributed autonomous sensor networks or communication relays equipped with acoustic or magnetic sensors, which may soon be used to detect, track or communicate with underwater vehicles in the littoral regions of the ocean or other aquatic environments. For these submersible sensors to function, they need an in-situ ballast system that allows the device to surface, where accumulated data can be transmitted via satellite or wireless networks, and the position of the sensor can be determined using global positioning technology. These links are impossible for submerged devices, as electromagnetic waves do not propagate well underwater and wired or acoustic-based systems have logistic and bandwidth shortcomings, respectively. However, each surfacing event requires large amounts of power, something unavailable for systems powered by microbial fuel cells.

To address this need for a ballast system with minimal power requirements, researchers in the Chemistry Division at the Naval Research Laboratory (NRL) in Washington, D.C., in conjunction with Southern Oregon University's Physics Department, have developed the zero power ballast control system. This autonomous buoyancy control system uses microbial energy from the fermentation of glucose to surface and submerge a water column sensor system, giving control of the system over to bacteria.

System Concepts and Design
The initial discussions of the system first took place in 2006 among NRL scientists, focusing on deploying microbial fuel cells in the water column. They were initially concerned about the small maximum power densities generated by microbial fuel cells, which at the time measured in microwatts per square centimeters of the electrode surface area, compared to the more than two watts needed to operate the device in the field. With these differences in mind, collaborators worked to minimize power consumption for all aspects of system operation.

Free body diagram for the general operation of the zero power buoyancy control, where Ff is buoyant force, Fg is gas force and Fw is weight force.

Modern advances in electronics have reduced power consumption in circuitry for data collection and sensor operation, meaning persistent aquatic devices can now potentially be powered by microbial fuel cells, as long as the power required to surface and submerge the device is eliminated. Moreover, because nonphotosynthetic microbes are capable of survival in dark environments, the use of microbes for power generation where solar power is not an option (i.e., underwater applications) is especially promising. Finally, the microbial utilization of nutrients from marine environments could potentially extend the operation of the power source in the device past the lifetime of batteries to more than two years.

The ability of a submerged device to surface periodically posts a significant challenge if this mode of operation is left to bacterial metabolism. To solve this problem, a straightforward solution that eliminated the need to power the buoyancy control system was designed and fabricated. The operation of the buoyancy control system can be understood by analyzing its free body diagram at different stages. In all cases, the total force acting on the device consists of three forces: the buoyancy of the float, the trapped gas and the weight. All three forces are oriented vertically. No other forces are present because all other components of the system are neutrally buoyant. If a net force points upward, the device will float. If the net forces points down, the device will sink. An NRL scientist then successfully designed and implemented a method that coupled the buoyancy control with biogenic gas production, creating a zero power consuming ballast control system.

Testing and Demonstration
The fully operational microbial zero power ballast control system was compared to an NRL prototype developed in conjunction with Nova Research Inc. (Alexandria, Virginia) on the same scale that was powered by 9.6-volt nickel-cadmium batteries. The difference in the overall power consumption between the two devices was greater than two watts. The operational life span of the powered buoyancy control system is ultimately limited by the amount of compressed gas stored or the battery capacity to power its pumps, valves or winches. The zero power ballast control system vents the gas continually and allows biogenic gas production rates to dictate the device's operation. To continue this article please click here.

Justin C. Biffinger is a research chemist at the U.S. Naval Research Laboratory in Washington, D.C. He holds an M.S. in inorganic chemistry from Bucknell University and a Ph.D. in organic chemistry from the University of Nebraska'Lincoln. His interests are developing alternative power sources and energy-harvesting biotechnology.

Bradley R. Ringeisen is the head of bioenergy and biofabrication section at the U.S. Naval Research Laboratory in Washington, D.C. He holds a Ph.D. in physical chemistry from the University of Wisconsin'Madison. His research interests are energy-harvesting biotechnology and biofabrication of engineered tissues.

Peter K. Wu is a professor of physics at the Southern Oregon University. He holds an M.S. and a Ph.D. in materials science from the University of Wisconsin'Madison. His research interests are tissue engineering and the applications of microbes and their byproducts.

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